1. Field of the Invention
The present invention relates to a method and a circuit for detecting the existence of a burst signal in received signals and outputting detection signal in response.
2. Description of the Related Art
In a low-speed burst modem, various controls, such as switching of an operation mode of a PLL (Phase Lock Loop) and switching of a bandwidth of the PLL, are carried out in order to realize a rapid pull-in performance during a period of receiving a carrier recovery training signal part located at the front of a burst signal. To effect those controls, the exact time of receipt of a preamble portion of the burst signal must be exactly known.
In a communication system such as a TDMA (Time Division Multiple Access) system, once synchronization is established, a time position of the preamble can be predicted in advance. Thus, the preamble information can be obtained from a synchronization control part of a receiver side.
In a communication system such as a packet signal transmission employing an aloha system or an SCPC (Single Channel Per Carrier) system equipped with voice-activation, where signals randomly generated are transmitted each time they are generated, a time position of a preamble a burst signal cannot be predicted in advance at the receiving side.
Therefore, the burst preamble must be independently detected in a burst demodulator itself to conduct the aforementioned controls. The burst preamble has, so far, been detected by envelope detection. More specifically, an envelope signal of baseband signals is generated by calculating, for example, .sqroot.I.sup.2 +Q.sup.2, where I and Q are magnitudes of baseband signals orthogonal to each other that are obtained by quadrature detection. The generated envelope signal is smoothed to eliminate noise components and a level of the smoothed envelope signal is compared with a threshold level. When the level of the envelope signal becomes higher than the threshold level, it is determined that a burst signal has been received.
The above conventional method of detecting the burst signal has the following shortcomings because existence of the burst signal is decided based on an amplitude of a received signal.
1) If the threshold level is set to too large value, detection fails, such that signals are determined to be nonexistent even though a signal does in fact exist. Conversely, if the threshold level is set to too small value, it is determined that a signal exists due to noise even though a signal does not exist, and therefore, a detection error occurs.
2) When an amplitude of a signal fluctuates, the frequency with which the above detection failure and detection error occurs increases. Therefore, a permitted range of fluctuation of the signal amplitude is strictly limited.
3) If an automatic gain control (AGC) operates while signals do not exist, gain increases, so that a noise level becomes equal to the signal level. Thus, the AGC cannot be used before the preamble is detected.
4) Error correction techniques in data transmission have progressed to the point that reproduction of data is possible even though a signal to noise ratio is 0 dB. Therefore, it is required that the burst signal detection is also possible in such a situation.